The present application relates to a bone anchoring element and to a stabilization device for bones, in particular for the spinal column, including such a bone anchoring element.
In the case of replacement of a severely injured or degenerated intervertebral disc or vertebra by fusion cages or bone segments stabilization devices using rigid metal rods are commonly used which are anchored in the neighboring vertebrae by polyaxial bone screws.
In specific clinical applications it is advantageous to maintain a certain mobility of the motion segments of the spinal column. In these cases, a dynamic stabilization system having bone anchoring elements and flexible rods are used. For example, US 2005/0085815 A1 and US 2007/0049937 A1 describe dynamic stabilization systems having a hollow metallic rod with a flexible section formed by a helix-shaped recess in the wall and a core provided in the hollow rod.
A dynamic stabilization device using polyaxial screws and an elastomer rod is described in EP 1 795 134 A1.
The known stabilization devices with flexible rods are suitable for the dynamic stabilization and motion control of the spinal column with respect to axial tension and compression forces.
Due to the anatomy of the spinal column, small sized implant constructs are required. Therefore, the flexible rods should have small outer diameters which makes it possible to design the receiving part of the polyaxial screw with a low profile and small overall dimensions.
In clinical cases of early degeneration or partial damages or injuries of intervertebral discs, the corresponding motion segments of the spinal column are subject to increased rotational movements and/or increased shearing forces. Such rotational movements and shearing and/or bending forces can cause strong pain. In addition, the flexible rods made of metal or elastomers may not be able to withstand higher forces for a long time due to their small diameter. In particular, shearing and rotational forces may cause an overload of the flexible rod.
Based on the foregoing, there is a need to provide a bone anchoring element and a stabilization device, in particular for the spinal column, which is suitable for cases in which increased rotational and shearing movements of the spinal column are present.